Project summary Novel mechanism for regulating macrophage polarization in sepsis Sepsis is a challenging clinical problem, and the pathogenesis is not clear. Macrophages (M?s) are dynamically modified by septic insult, with different cell subsets arising during early and late stages of sepsis development. M?s are heterogeneous in tissues; they polarize into M1 (pro-inflammatory) and M2 (anti-inflammation) upon environmental stimuli. How M? polarization occurs in response to sepsis is largely unknown. Our long-term goal is to elucidate the molecular mechanism underlying M? polarization in sepsis pathogenesis. Here, we propose to test the original idea that Sirt6 drives M? polarization in septic lung. Sirt6 is a histone deacetylase family member that has been shown to promote TNF-? production and maintain M? glucose homeostasis. Thus, Sirt6 modification of histone marks may be involved in the epigenetic regulation of M? polarization. Moreover, a septic insult alters the M? microRNA (miRNA) profile, important mediators of post-transcriptional gene expression, which are increasingly recognized as being critical to myeloid cell development. Whether miRNA regulates Sirt6 in M? polarization in response to septic insult is unknown. Our central and innovative hypothesis is that sepsis-associated changes in miRNA regulate the expression of Sirt6, which in turn targets histone marks to drive expression of genes involved in M? polarization. In preliminary studies, we found that (1) sepsis induces Sirt6 and phenotypic changes in pulmonary M?s; (2) Sirt6 is differentially expressed in BMDM M1 and M2 cells and determine their cytokine phenotypes; (3) A typical miRNA regulates Sirt6 expression in M? polarization; and (4) an acetylated histone mark is differentially expressed in M1 and M2 in an opposite pattern to that of Sirt6. Based on these evidences, we proposed to determine whether Sirt6 controls pulmonary M? polarization in sepsis and if yes, determine the underlying mechanism. In Aim 1, we will set up a myeloid-specific Sirt6 knockout mice strain to define how deletion of Sirt6 affects pulmonary M? phenotypes during early and late sepsis. We will also determine the association of Sirt6 with pulmonary M?s in septic lung by flow cytometry sorting and Western blot analysis. In Aim 2, we will explore the roles of miRNA and histone acetylation in Sirt6 regulation of M? polarization. We expect that the proposed research will identify Sirt6 as a regulator of M? polarization and characterize a miRNA-Sirt6-H3K27ac axis as a novel mechanism that drives pulmonary M? activation in sepsis pathogenesis. Our findings may lead to the identification of novel targets for the development of new therapies that address the pathophysiology of sepsis.